The atuonomic nervous system plays ans important role in influencing cardiac function in the mature mammal; however its significance in te pre- and preinatal period is not clear. In dealing with this problem most studies have been pharmacological or physiological in nature and have aimed at assessing the ability of the heart to respond to various autonomic drugs, blocking agents, or other stimuli. Few anatomical and histochemical studies have been undertaken to examine the maturation, distribution and relationships of autonomic nerve elements to cardiac muscle cells in the hearts of fetal animals so as to establish their potential for participation in circulatory responses. The aims of the proposed research are to study: 1. the growth and maturation of autonomic nerves in the hearts of fetal rabbits. 2. the time of appearance of catecholamines and acetylcholinesterase in autonomic nerves and changes of these substances with increasing ages. 3. the establishment of functional relationships between these nerves and both the regular cardiac and specialized conduction cells. All work will be done on fetal and postnatal rabbits of known ages. Electron microscopy and histochemical techniques for localizing catecholamine and acetylcholinesterase in autonomic nerves and changes of these substances with increasing age; 3. the establishment of functional relationships between these nerves and both the regular cardiac and specialized conduction cells. All work will be done on fetal and postnatal rabbits of known ages. Ellctron microscopy and histochemical techniques for localizing catecholamines and acetylcholinesterase in automonic nerves will be employed. Biochemical assays will be done to determine levels of norepinephrine in the heart. This research should add basic information to aid our understanding of neural influences on the heart during ontogenesis.